Question

The schematic below describes the status of the lac operon in the absence of lactose. Which one of the following conditions happens when lactose is present in the cell?

• A. Lactose binds to $P^i$ and stops the transcription of $i$.
• B. Lactose is converted to allolactose, which binds to $P^{lac}$ and results in the displacement of the repressor from $O$.
• C. Lactose is converted to allolactose, which binds to the repressor protein and prevents its interaction with $O$.
• D. Lactose has no effect on the status of the lac operon.

Answer 1

The correct option is C Lactose is converted to allolactose, which binds to the repressor protein and prevents its interaction with $O$.

When lactose is absent, repressor protein is transcribed. It binds to the operator region and prevents the transcription of Z, Y and A genes. However, in presence of lactose, lactose is first converted to allolactose. This allolactose binds to repressor protein. This binding prevents the interaction of repressor protein with operator region. Hence, the transcription and translation of the Z, Y and A genes will take place.

Thus, the correct answer is option C.